The present invention relates to a method that provides fiber reinforcement to a thermoplastic weld to alleviate residual tensile strain and resulting cracking.
The exponential decay of the strength of magnetic fields dictates that, in induction welding processes, the structure closest to the induction coil will be the hottest, since it experiences the strongest field. Therefore, it is difficult to obtain adequate heating at the bond line between two graphite or carbon fiber reinforced resin matrix composites relying on the susceptibility of the fibers alone as the source of heating in the assembly. For the inner plies to be hot enough to melt the resin, the outer plies closer to the induction coil and in the stronger magnetic field are too hot. The matrix resin in the entire piece of composite melts. The overheating results in porosity in the product, delamination, and, in some case, destruction or denaturing of the resin. To avoid overheating of the outer plies and to insure adequate heating of the inner plies, a susceptor of significantly higher conductivity than the fibers is used to peak the heating selectively at the bond line of the plies when heating from one side. An electromagnetic induction coil on one side of the assembly heats a susceptor to melt and cure a thermoplastic resin (also sometimes referred to as an adhesive) to bond the elements of the assembly together. Often the current density in the susceptor is higher at the edges of the susceptor than in the center because of the nonlinearity of the coil. This problem typically occurs when using a cup core induction coil like that described in U.S. Pat. No. 5,313,037 and can result in overheating the edges of the assembly or underheating the center, either condition leading to inferior welds because of non-uniform curing. It is necessary to have an open or mesh pattern in the susceptor to allow the resin to bond between the composite elements of the assembly when the resin heats and melts.
Three major joining technologies exist for aerospace composite structure: mechanical fastening; adhesive bonding; and welding. Both mechanical fastening and adhesive bonding are costly, time consuming assembly steps that introduce excess cost even if the parts that are assembled are fabricated from components produced by an emerging, cost efficient process. Mechanical fastening requires expensive hole locating, drilling, shimming, and fastener installation, while adhesive bonding requires complicated surface pretreatments.
In contrast, thermoplastic welding, which eliminates fasteners, features the ability to join thermoplastic composite components at high speeds with minimum touch labor and little, if any, pretreatments. In our experience, the welding interlayer, called a susceptor, also can simultaneously take the place of shims required in mechanical fastening. As such, composite welding holds promise to be an affordable joining process. For xe2x80x9cweldingxe2x80x9d thermoplastic and thermoset composite parts together, the resin that the susceptor melts functions as a hot melt adhesive. If fully realized, the thermoplastic-thermoset bonding will further reduce the cost of composite assembly.
There is a large stake in developing a successful induction welding process. Its advantages versus traditional composite joining methods are:
reduced parts count versus fasteners
minimal surface preparation, in most cases a simple solvent wipe to remove surface contaminants
indefinite shelf life at room temperature
short process cycle time, typically measured in minutes
enhanced joint performance, especially hot/wet and fatigue
permits rapid field repair of composites or other structures.
There is little or no loss of bond strength after prolonged exposure to environmental influences.
U.S. Pat. No. 4,673,450 describes a method to spot weld graphite fiber reinforced PEEK composites using a pair of electrodes. After roughening the surfaces of the prefabricated PEEK composites in the region of the bond, Burke placed a PEEK adhesive ply along the bond line, applied a pressure of about 50-100 psi through the electrodes, and heated the embedded graphite fibers by applying a voltage in the range of 20-40 volts at 30-40 amps for approximately 5-10 seconds with the electrodes. Access to both sides of the assembly is required in this process which limits its application.
Prior art disclosing thermoplastic welding with induction heating is illustrated by U.S. Pat. Nos. 3,966,402 and 4,120,712. In these patents, the metallic susceptors used are of a conventional type having a regular pattern of openings of traditional manufacture. Achieving a uniform, controllable temperature in the bondline, which is crucial to preparing a thermoplastic weld of adequate integrity to permit use of welding in aerospace primary structure, is difficult with those conventional susceptors, as we discussed and illustrated in our copending U.S. patent application Ser. No. 08/068,520.
Thermoplastic welding is a process for forming a fusion bond between two faying thermoplastic faces of two parts. A fusion bond is created when the thermoplastic on the surface of the two parts is heated to the melting or softening point and the two surfaces are brought into contact, so that the molten thermoplastic mixes, and the surfaces are held in contact while the thermoplastic cools below the softening temperature.
Simple as the thermoplastic welding process sounds, and easy as it is to perform in the laboratory on small pieces, it becomes difficult to perform reliably and repeatably in a real factory on full-scale parts to build a large structure such as an airplane wing box. The difficulty is in getting the proper amount of heat to the bondline without overheating the entire structure, and also in achieving intimate contact of the faying surfaces of the two parts at the bondline during heating and cooling despite the normal imperfections in the flatness of composite parts, thermal expansion of the thermoplastic during heating to the softening or melting temperature, flow of the thermoplastic out of the bondline under pressure, and then contraction of the thermoplastic in the bondline during cooling.
One technique for getting heat to the bondline in a thermoplastic assembly to be welded together is to include a conductive layer or article, known as a xe2x80x9csusceptorxe2x80x9d, between the two surfaces to be welded, and to heat the susceptor by resistive heating so that the susceptor functions as a bondline heat source to melt or soften the thermoplastic at the bondline for fusion of the faying surfaces of the composite components to be joined. The electric current for heating the susceptor can be in the form of eddy currents generated inductively, as taught for example by U.S. Pat. Nos. 3,395,261 and 4,978,825, or it can be conducted directly to the susceptor through tabs or the like as shown in U.S. Pat. No. 5,313,034.
In U.S. patent application Ser. Nos. 08/286,360 and 08/068,520, we described a tailored susceptor for approaching the desired temperature uniformity. This susceptor relied upon carefully controlling the geometry of openings in the susceptor (both their orientation and their spacing) to distribute the heat evenly. For example, we suggested using a regular array of anisotropic, diamond shaped openings with a ratio of the length (L) to the width (W) greater than 1 to provide a superior weld over that achieved using a susceptor having a similar array, but one where the L/W ratio was one. By changing the length to width ratio (the aspect ratio) of the diamond-shaped openings in the susceptor, we achieved a large difference in the longitudinal and transverse conductivity in the susceptor, and, thereby, tailored the current density within the susceptor. A tailored susceptor having openings with a length (L) to width (W) ratio of 2:1 has a longitudinal conductivity about four times the transverse conductivity. In addition to tailoring the shape of the openings to tailor the susceptor, we altered the current density in regions near the edges by increasing the foil density (i.e., the absolute amount of metal). Increasing the foil density along the edge of the susceptor increases the conductivity along the edge and reduces the current density and the edge heating. We increased foil density by folding the susceptor to form edge strips of double thickness or by compressing openings near the edge of an otherwise uniform susceptor. We found these susceptors difficult to reproduce reliably. Also, their use forced careful placement and alignment to achieve the desired effect.
The tailored susceptor for our earlier application was designed to use with the cup core of U.S. Pat. No. 5,313,037. With this coil, the magnetic field is strongest near the edges because the central pole creates a null at the center. Therefore, the susceptor is designed to counter the higher field at the edges by accommodating the induced current near the edges. The high longitudinal conductivity encourages induced currents to flow longitudinally.
Our selvaged susceptor for thermoplastic welding which is described in U.S. patent application Ser. No. 08/314,027 controls the current density pattern during eddy current heating by an induction coil to provide substantially uniform heating to a composite assembly and to insure the strength and integrity of the weld in the completed part. This susceptor is particularly desirable for welding ribs between prior welded spars using an asymmetric induction coil, because, with that coil, it provides a controllable area of intense, uniform heating, a trailing region with essentially no heating, and a leading region with minor preheating.
The power (P) or power density which the susceptor dissipates as heat follows the well-known equation for power loss in a resistor: P=(J2)(R) wherein J is the eddy current (or its density) and R is the impedance (i.e., resistance) of any segment of the eddy path. The heating achieved directly corresponds to the power (or power density).
We achieve better performance (i.e., more uniform heating) in rib welding by using a salvaged susceptor having edge strips without openings. The resulting susceptor, then, has a center portion with a regular pattern of opening and solid foil edges, which we refer to as selvage edge strips. We embed the susceptor in a thermoplastic resin to make a susceptor/resin tape that is easy to handle and to use in performing the composite pieces prior to welding. Also, we have discovered that, with a selvaged susceptor, the impedance of the central portion should be anisotropic with a lower transverse impedance than the longitudinal impedance. Here, the L/W ratio of diamond shaped openings should be less than or equal to one. That is, unlike our tailored susceptor of U.S. patent application Ser. No. 08/286,360, L for the selvaged susceptor should be less than W. With this new selvaged susceptor in the region immediately under the asymmetric induction work coil described in U.S. patent application Ser. No. 08/349,697, we encourage the current to flow across the susceptor to the edges where the current density is lowest and the conductivity, highest.
Generally, we form the selvaged susceptor somewhat wider than normal so that the selvage edge strips are not in the bondline. We remove the selvage edge strips after forming the weld, leaving only a perforated susceptor foil in the weld. This foil has a relatively high open area fraction.
Significant effort has been expended in developing inductor and susceptor systems to optimize the heating of the bondline in the thermoplastic assemblies to be welded. Induction coil structures and tailored susceptors have now been developed that provide adequate control and uniformity of heating of the bondline, but a big hurdle remaining to perfecting the process to the point of practical utility for producing large scale aerospace-quality structures in a production environment is the aspect of the process dealing with the control of the surface contact of the faying surfaces of the two parts to be welded together, and the timing, intensity, and schedule of heat application so the material at the faying surfaces is brought to and maintained within the proper temperature range for the requisite amount of time for an adequate bond to form, and is maintained in intimate contact while the melted or softened material hardens in its bonded condition.
Large scale parts such as wing spars and ribs, and the wing skins that are bonded to the spars and ribs, are typically on the order of 20-30 feet long at present, and potentially can be hundreds of feet in length when the process is perfected for commercial transport aircraft. Parts of this magnitude are very difficult to produce with perfect flatness. Instead, the typical part will have various combinations of surface deviations from perfect flatness, including large scale waviness in the direction of the major length dimension, twist about the longitudinal axis, dishing or sagging of xe2x80x9cIxe2x80x9d beam flanges, and small scale surface defects such as asperities and depressions. These irregularities interfere with full surface area contact between the faying surfaces of the two parts and actually result in surface contact only at a few xe2x80x9chigh pointsxe2x80x9d across the intended bondline. Additional surface contact can be achieved by applying pressure to the parts to force the faying surfaces into contact, but full intimate contact is difficult or impossible to achieve in this way. Applying heat to the interface by electrically heating the susceptor in connection with pressure on the parts tends to flatten the irregularities further, but the time needed to achieve full intimate contact with the use of heat and pressure is excessive, can result in deformation of the top part, and tends to raise the overall temperature of the xe2x80x9cIxe2x80x9d beam flanges to the softening point, so they begin to yield or sag under the application of the pressure needed to achieve a good bond.
Our multipass thermoplastic welding process described in U.S. patent application Ser. No. 08/367,546 enables a moving coil welding process to produce continuous or nearly continuous fusion bonds over the full area of the bondline to yield very high strength welds reliably, repeatably and with consistent quality. This process produces improved low cost, high strength composite assemblies of large scale parts, fusion bonded together with consistent quality, and uses a schedule of heat application that maintains the overall temperature of the structure within the limit in which it retains its high strength, so it requires no internal tooling to support the structure against sagging which otherwise could occur above the high strength temperature limit. The process also produces nearly complete bondline area fusion on standard production composite material parts having the usual surface imperfections and deviations from perfect flatness, while eliminating fasteners and the expense of drilling holes, inspecting the holes and the fasteners, inspecting the fasteners after installation, sealing between the parts and around the fastener and the holes; reducing mismatch of materials; and eliminating arcing from the fasteners.
In the process, an induction heating work coil is passed multiple times over a bondline while applying pressure in the region of the coil to the components to be welded, and maintaining the pressure until the resin hardens. The resin at the bondline is heated to the softening or melting temperature with each pass of the induction work coil and pressure is exerted to flow the softened/melted resin in the bondline and reduce the thickness of the bondline while improving the intimacy of the faying surface contact with each pass to militate for complete continuity of bond. The total time at the softened or melted condition of the thermoplastic in the faying surfaces is sufficient to attain deep inter diffusion of the polymer chains in the materials of the two faying surfaces throughout the entire length and area of the bondline, thereby producing a bondline of improved strength and integrity in the completed part, but the total time of the faying surfaces at softened temperature is in separate time segments which allows time for the heat in the interface to dissipate without raising the temperature of the entire structure to the degree at which it loses its strength and begins to sag, so the desired shape and size of the final assembly is maintained.
The method of the present invention uses a structural susceptor to include fiber reinforcement within the weld resin to alleviate residual tensile strain and to suppress cracking otherwise present in an unreinforced weld. The susceptor includes alternating layers of thin film thermoplastic resin sheets and fiber reinforcement (usually woven fiberglass fiber comprising at least about 30 vol. % of the weld) sandwiching the conventional metal susceptor that is embedded in the resin. Conventional susceptors are described U.S. patent application Ser. Nos. 8/286,360 and 08/314,027. While the number of total plies in this structural susceptor is usually not critical, we prefer to use at least two plies of fiber reinforcement on each side of the susceptor.
The present invention also relates to a method for gap filling between the welded composite laminates which tailors the thickness (number of plies) in the structural susceptor to fill the gaps, thereby eliminating costly profilometry of the faying surfaces and the inherent associated problem of resin depletion at the faying surfaces caused by machining the surfaces to have complementary contours. Standard manufacturing tolerances produce gaps as large as 0.120 inch, which is too wide to create a quality weld using conventional susceptors.
We can easily tailor the thickness of the structural susceptor to match the measured gap by scoring through the appropriate number of plies of resin and fiber reinforcement and peeling them off. In doing so, a resin side layer will be on both faying surfaces and this layer should insure better performance from the weld.